Windowed package having embedded frame

ABSTRACT

An integrated circuit (IC) package includes a mold compound, a die, and a window. The mold compound has a frame embedded within it. The frame has a top surface, a bottom surface, and a top-to-bottom opening therein. The die is attached to the mold compound, wherein the embedded frame lies below a periphery of the die. The window is attached to the mold compound and located above the die to allow light to reach the die.

[0001] This application is a continuation of Ser. No. 09/546,225, filedApr. 10, 2002, which is a divisional of Ser. No. 09/219,186, filed onDec. 21, 1998, which is a continuation-in-part of application Ser. No.09/172,734, filed on Oct. 13, 1998, entitled “Image Sensor Mounted byMass Reflow.”

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The described invention relates to the field of integratedcircuit packages. In particular, the invention relates to an integratedcircuit package capable of being mounted to a circuit board via a massreflow process.

[0004] 2. Description of Related Art

[0005] A windowed integrated circuit package is used for variousapplications in which an integrated circuit is illuminated or irradiatedby light or other radiation sources located outside the integratedcircuit package. An image sensor is one use of a windowed integratedcircuit package.

[0006] For example, a photodiode array may be placed within a windowedintegrated circuit package. The photo-detector array provides an imagedata output based upon the light incident on the photo-detector array.The photo-detector array may be used for capturing images or for otherimage reproduction applications. A color filter array (CFA) material isused with the photo-detector to filter the light impinging on the imagesensor to allow for the formation of full color images. Each filterallows a predetermined color of light to reach a correspondingphoto-detector, thus determining what color light will be sensed by thephoto-detector. By grouping sets of light sensors together, theintensity and color of light reaching an area can be determined.

[0007] Integrated circuit (IC) packages are mounted on circuit boards byvarious techniques including mass reflow and manual and hot barsoldering of the package to the circuit board. Manual soldering and hotbar soldering, however, are relatively slow and expensive processes.

[0008] Mass reflow board mounting is a faster, automated process. Massreflow refers to one of several different techniques that raise thetemperature of the IC package to approximately 215 to 225° C. At theseelevated temperatures, solder residing on pads of the integrated circuitboard melts and adheres to leads on the IC package. After the soldercools, the IC package remains firmly coupled to the solder pads. Massreflow includes infrared, convection, and vapor phase techniques.

[0009] Non-ceramic packages such as windowed plastic packages are moredesirable than ceramic packages because they are of lower cost thancorresponding ceramic windowed packages. However, until recently,standard windowed plastic packages tested on the mass reflow processexhibited problems such as cracked lids, delamination of the die fromthe die attach, and lid-sealant separation due to thermal expansionmismatch between the plastic and glass window. Until recently, thesewindowed plastic packages were mounted to circuit boards usingtechniques such as manual soldering that keep the package bulk fromreaching the elevated temperatures of the mass reflow process.

[0010] Co-pending U.S. patent application Ser. No. 09/172,710, entitled“Mass Reflowable Windowed Non-Ceramic Package,” assigned to IntelCorporation and Kyocera Corporation describes a windowed non-ceramicpackage that does meet the thermal requirements for mass reflow boardmounting. However, a way of reducing internal stresses is desirable.This would allow extended bake times to be reduced or eliminated, andlarger package sizes to be mass reflowed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a cross sectional block diagram of the windowed quadflat pack (QFP) package 10 as modified by the Applicants.

[0012]FIG. 2A shows a schematic diagram of one embodiment of the packagelid, including the ceramic frame and glass window.

[0013]FIG. 2B shows a side view of the package lid.

[0014]FIG. 2C shows another side view of the package lid.

[0015]FIG. 3A shows a schematic diagram of one embodiment of the entireIC package.

[0016]FIG. 3B shows a side view of the entire IC package.

[0017]FIG. 4 shows one embodiment of the process for attaching a dieonto a windowed non-ceramic package.

[0018]FIG. 5 shows a cross sectional view of one embodiment of the moldcompound including an embedded frame.

[0019]FIGS. 6 and 7 show modeling of warpage patterns at 225° C. of anIC package with an embedded frame and without an embedded frame,respectively.

[0020]FIG. 8 shows one embodiment of an embedded frame that has crossbars that join the sides of the embedded frame.

[0021]FIGS. 9A and 9B show one embodiment of a mold that can be used toembed a frame within the mold compound.

[0022]FIG. 10 shows an imaging system comprising an image sensorattached to a circuit board via a mass reflow process.

DETAILED DESCRIPTION

[0023] An improvement to a windowed non-ceramic integrated circuit (IC)package capable of being mounted via a mass reflow process is disclosed.A frame is embedded in the mold compound of the IC package. This reducesthe stresses internal to the IC package when raised to the hightemperatures associated with mass reflow.

[0024] Applicants have found that modifying a windowed plastic QFPpackage available through Kyocera Corporation based in Kyoto. Japanallows the plastic package to withstand the mass reflow process withoutthe lid separating from the molded package or the die delaminating fromthe molded package. Additionally, the non-ceramic IC package may becombined with a CFA material with high temperature stability to producean image sensor that maintains its color performance despite beingexposed to a mass reflow process.

[0025] The following section describes the IC package capable of beingmass reflowed that is also described in co-pending U.S. patentapplication Ser. No. 09/172,710, entitled “Mass Reflowable WindowedNon-Ceramic Package,” assigned to Intel Corporation and KyoceraCorporation. The subsequent section describes the improvement of addingan embedded frame into the mold compound of the IC package.

[0026] Mass Reflowable Windowed Non-Ceramic Package

[0027]FIG. 1 shows a cross sectional block diagram of a windowed QFPpackage 10 that is mass reflowable. A non-ceramic molded package 12makes up the package body. For one embodiment, the non-ceramic moldedpackage is made with a low-moisture plastic, such as a low-moisture moldcompound of ortho-cresol-novolac developed by Kyocera Corporation. Forone embodiment, depressions 22 indicate where ejector pins were used toremove the molded package after being formed. Appendix 1 includes anexample of the material characteristics of a low-moisture mold compoundof Kyocera Corporation.

[0028] Die attach 14 is used to hold the die 16 in place. For oneembodiment, the die attach 14 is a low rigidity epoxy such as asilver-filled epoxy manufactured by Ablestik Electronic Materials andAdhesives, based in Rancho Dominguez, Calif.

[0029] Wire bonds 18 attach the die 16 to a lead frame 20. The dieattach 14 is selected to withstand the elevated temperatures of the massreflow process. Delamination of the die 16 from the die attach 14 ormolded package 12 may be a problem during mass reflow. Applicants havedetermined that a two step cure process for the die attach, as will bediscussed with respect to FIG. 3, solves this problem.

[0030] A lid 30 seals the molded package. For one embodiment, the lid 30comprises a ceramic frame 32 made of alumina. The ceramic frame 32 holdsa transparent window. For one embodiment, the ceramic frame 32 includesa recessed ledge within which a glass window 34 resides. For oneembodiment, the molded package 12 and the ceramic frame 32 are sealedusing a bis-phenol A type epoxy. The epoxy seal may also be used to sealthe ceramic frame 32 to the glass window 34. Appendix 2 summarizes thecharacteristics of a bis-phenol A-type sealant that is suitable for usewith the present invention.

[0031] The modified windowed package is particularly suited for, but notlimited to, Complementary Metal Oxide Semiconductor (CMOS) image sensorsbecause their die size are relatively large (they can exceed 240 mils by240 mils). An embodiment of the package suitable for an image sensorincludes a window having a slightly larger area than that of thelight-sensitive portion of the die.

[0032] For one embodiment, the window is approximately 1.2 times thearea of the light-sensitive portion of the die. The window size,however, varies depending on its distance away from the die. FIGS. 2 and3 show schematic diagrams of one embodiment of the lid and moldedpackage.

[0033] FIGS. 2A-2C show a schematic of one embodiment of the package lid30, including the ceramic frame 32 and glass window 34. The firstdimension is in mils, the dimension in parentheses is in millimeters.For one embodiment, a glass window 34 is seated into a recessed ledge 40in the ceramic frame 32.

[0034] FIGS. 3A-3B show a schematic of one embodiment of the entire ICpackage 50 in accordance with present invention. The first dimensionindicated for an element is in inches, the second dimension (inparentheses) is in millimeters. Although the embodiment shown includes aparticular type of lead frame (quad flat pack—QFP), other types of leadframes may be employed. Moreover, other packages may be used, includingleadless packages such as ball grid array (BOA) packages, leadless chipcarrier (LCC) packages, and leaded packages such as dual in-line package(DIP), and so forth.

[0035]FIG. 4 shows one embodiment of the process for attaching a dieinto a windowed non-ceramic package. At step 202, die attach isdispensed on the molded package. For one embodiment, the die attach iscomprised of a low rigidity epoxy such as a silver-filled epoxy, aspreviously stated.

[0036] The process continues at block 204, at which the die is scrubbed,or moved back and forth while pressure is applied to firmly attach thedie to the die attach. Good adhesion of the die to a smooth surface ofthe molded package is achieved without plating the back-side of the diewith gold.

[0037] The die attach is cured at block 206. It is important toeliminate voids in the die attach, which may cause delaminationproblems. It has been found that a two stage cure process works betterthan a one stage cure process for eliminating voids in the die attach.For one embodiment, the die attach is baked at approximately 100° C. forapproximately one hour, then the die attach is baked at approximately150° C. for approximately another hour.

[0038] At block 208, wire bonds are attached between the die and thelead frame of the molded package.

[0039] At block 212, the lid is attached to the molded package. For oneembodiment, the lid comprises the glass window 34 attached to theceramic frame 32 with a his-phenol A type epoxy. The epoxy is cured bybaking. For one embodiment, the curing is performed by raising thetemperature to approximately 150° C. for approximately 70 minutes. Forone embodiment, the lid is attached to the molded package using the sameepoxy that was used to attach the glass window 34 to the ceramic frame32, and the epoxy is cured by also raising the temperature toapproximately 150° C. for approximately 70 minutes.

[0040] For one embodiment, the above steps are performed using laminarflow hoods in a clean room that meets level class sub 100, in whichthere is less than 100 particles of contamination of one micron or lessper cubit meter. This helps to prevent particle contamination of the dieduring assembly.

[0041] For one embodiment, the window has a scratch-dig specification of20 microns. The scratch-dig specification denotes the largest allowabledefect in the glass. A larger defect may interfere with the imagingperformance of the image sensor.

[0042] In order to reduce moisture in the sealed package prior toperforming the mass reflow process, an extended bake cycle may be usedjust prior to bagging. For one embodiment, the sealed package is bakedat 125° C. for 48 hours, then it is vacuum sealed in a moisture barrierbag for storage or shipment. This allows the sealed package to meet theInstitute for Interconnecting and Packaging Electronic Circuits (IPC)level 4 surface mount requirements. (The IPC is a trade associationrepresenting over 2300 companies in the electronic interconnectionindustry worldwide.)

[0043] When the sealed package is ready to be mounted, it is removedfrom the bag and mounted to a circuit board using a mass reflow process,as shown at block 214. There are various types of mass reflow processes.For one embodiment, an IR/convection mass reflow process is employedthat conforms to the following:

[0044] peak package body temperature of approximately 225° C.;

[0045] time above 215° C. is approximately 30 seconds;

[0046] time above 183° C. is approximately 140 seconds.

[0047] The windowed non-ceramic package is able to withstand the abovemass reflow process without the lid separating from the molded packageor die detaching from the molded package.

[0048] Mass Reflowable Windowed Non-Ceramic Package with Embedded Frame

[0049]FIG. 5 shows a cross sectional view of one embodiment of the moldcompound including an embedded frame 300. The mold compoundsubstantially surrounds the frame. (It may or may not be surrounded onthe bottom surface.) For one embodiment, the frame is made of ceramic tomatch the CTE of the ceramic frame bordering the window of the ICpackage. In another embodiment, the embedded frame is made of materialhaving a lower CTE than that of the mold compound. For example, theframe may comprise a copper/tungsten alloy or alloy-42 (an alloycomprising 42% nickel and 58% iron), which is frequently used in leads.

[0050] Because the stresses internal to an IC package are difficult tomeasure, the stresses have been modeled by finite element modeling,which is commonly used as an engineering validation tool of mechanicaldesigns. Table 1 shows exemplary values of the CTEs used in the modelingfor the mold compound, the die attach, the silicon die, the windowsealant, the window, the ceramic frame, and the leads. Table 1 alsoshows values of Young's modulus (an indication of stiffness of thematerial) and Poisson's ratio (an indicative ratio of deformation in onedirection when stretched in an orthogonal direction) which were used inthe modeling. TABLE 1 Materials properties at Different TemperaturesModulus, kN/mm² CTE, ppm/° C. 25 150 155 225 25 150 155 225 Poisson'sComponents ° C. ° C. ° C. ° C. ° C. ° C. ° C. ° C. ratio*** Mold 14.414.4 2.1 2.1 11.4 11.4 48.1 48.1 .23 Compound* Die Attach* 8.0 .08 .08.08 25 25 130 130 .3 Silicon Die 130 130 130 130 2.6 3.2 3.2 3.6 .23Window 4.2 4.2 .045 .045 62.1 62.1 186 186 .35 Sealant** Glass 72.9 72.972.9 72.9 6.8 7.5 7.5 7.9 .208 Window Ceramic 280.5 275.6 275.6 272.65.74 6.76 6.76 7.38 .25 Window Frame Lead (alloy 145 145 145 145 4.5 4.54.5 4.5 .3 42)

[0051] According to the above table, the materials properties of moldcompound, die attach and sealant remains constant from 25 to its Tg,then change to a different set of values from Tg+5° C. to 225° C. TheSilicon properties reflect those of [110] directions.

[0052] Table 2 is a summary of the modeled results of the reduction ofinternal stresses of an IC package using an embedded frame comprisingalloy-42. The modeled results show that stresses in an IC package havingthe embedded frame are reduced significantly for both of the epoxyinterfaces between the backside die and the molded package and betweenthe lid and the molded package. This reduction in stress makes it easierto attach larger package sizes via mass reflow since larger packagesizes are more susceptible to internal stresses at temperature extremes,such as at mass reflow temperatures. TABLE 2 Maximum Von Mises Max VonMises Warpage in Package Stress in Die Attach, Stress in Window Bottom*,μm MPa Seal, MPa Temperature ° C. −65 223 225 −65 25 225 −65 25 225Without Embedded −45.4 −26.4 +60.0 179.3 104.3 16.4 129.8 76.9 5.8 FrameWith Embedded −23.4 −13.6 +13.3 136.5 79.4 4.8 133.2 77.4 4.7 FrameDifference, % 48.5 48.5 77.8 23.9 23.9 70.3 −2.6 −0.65 19.0

[0053]FIGS. 6 and 7 show modeled warpage patterns at 225° C. for an ICpackage with an embedded frame and without an embedded frame,respectively. Warpage is seen to be significantly reduced using anembedded frame.

[0054] The embedded frame may take on a variety of shapes. However, toreduce costs, a simple design such as a square loop or a miniatureversion of the window frame 32 of FIG. 2 may be used. Applicant hasfound the best results by the embedded frame overlapping with theperiphery of the die as shown in FIGS. 5 and 6.

[0055] For one embodiment, the embedded frame 300 may include cross bars302 joining the sides 304 for additional support, as shown in FIG. 8. Inthe miniature version of the window frame 32, a ledge holds the embeddedframe firmly in the mold compound. A more complicated shape of frame mayalso be used (for instance a honeycomb pattern), but the additional costof the frame should be low enough such that the overall package remainsless expensive than a flail ceramic package.

[0056] In another embodiment, a full glass window without a window frameis attached to the mold compound. An embedded frame reduces the stressesinvolved during the mass reflow process. For example, an embedded framemade of alloy-42 (CTE ˜4.5 ppm/C), a copper/tungsten alloy (CTE6.5˜ppm/C), or a ceramic frame (CTE ˜7.4 ppm/C) may be employed.

[0057] The Mold Process

[0058]FIGS. 9A and 9B show one embodiment of a mold that can be used toembed a frame within the mold compound. FIG. 9A shows a cross sectionalside view of the mold showing the placement of embedded frame 300 withinthe mold cavity 340. FIG. 9B shows a top view of the mold cavitycorresponding to the dotted line 350 of FIG. 9A that looks down upon theembedded frame 300.

[0059] In this embodiment, stoppers 342 are used to hold the embeddedframe 300 in place. Runners and a gate 360 located on one side of themold cavity provides a source of the mold compound. The location of therunners and gate to the mold cavity 340 is designed such that the flowof mold compound will apply pressure over the top surface of theembedded frame and hold the embedded frame down in the mold cavity 340.A vent 362 on the opposite side of the mold allows air to be expelled asthe mold compound is supplied into the mold cavity 340. A lead frame 370is also embedded when the mold compound is supplied to the mold cavity340.

[0060] For one embodiment, multiple molded packages are made at the sametime. The molded packages are joined together end-to-end in a strip, asis well known. The molded packages are separated from another in anotherprocess step.

[0061] Imaging System

[0062]FIG. 10 shows an imaging system 400 comprising an image sensorutilizing the IC package with an embedded frame. The image sensor isattached to a circuit board via a mass reflow process. The image sensor410 is employed as part of a camera, silicon eye, or other image device.Typically, the image sensor is electrically coupled to an imageprocessor 420 and a memory 430. The imaging system may also includeinterconnect circuitry 440 for communicating with other systems, such asa host computer system or other output device. The imaging system mayalso include a lens system (not shown) to focus the light on the imagesensor, as is well-known in the art.

[0063] The ability to attach the image sensor via the mass reflowprocess reduces costs and speeds up the manufacturing process. It alsomay provide a more reliable connection than manual soldering methods.

[0064] Thus, an IC package with an embedded frame capable of beingmounted to a circuit board via a mass reflow process is disclosed. Thespecific arrangements and methods described herein are merelyillustrative of the principles of this invention. Numerous modificationsin form and detail may be made without departing from the scope of thedescribed invention. For example, with reductions due to the embeddedframe of internal stresses at both the epoxy interfaces between thebackside die and molded package and between the lid and molded package,extended bake cycles may not be needed. Although this invention has beenshown in relation to a particular embodiment, it should not beconsidered so limited. Rather, the described invention is limited onlyby the scope of the appended claims.

What is claimed is:
 1. An integrated circuit (IC) package comprising: amold compound, the mold compound having a frame embedded therein, saidembedded frame having a top surface, a bottom surface, and atop-to-bottom opening therein; a die attached to the mold compoundwherein the embedded frame lies below a periphery of the die; and awindow attached to the mold compound and located above the die to allowlight to reach the die.
 2. The IC package of claim 1, wherein theembedded frame substantially comprises ceramic.
 3. The IC package ofclaim 1, wherein the embedded frame substantially comprises an alloy. 4.The IC package of claim 3, wherein the embedded frame substantiallycomprises alloy-42.
 5. The IC package of claim 1 further comprising: awindow frame bordering the window, the window frame having a CTE smallerthan that of the mold compound.
 6. The IC package of claim 5, whereinthe window frame is made of the same material as that of the embeddedframe.
 7. The IC package of claim 1, wherein the frame lies below aperiphery of the die.
 8. An integrated circuit (IC) package comprising:a mold compound having a ceramic frame embedded therein; a die having aperiphery, wherein the frame lies below the periphery of the die; and awindow having a ceramic frame that is attached to the mold compound andis located above the periphery of the die.
 9. A method of making an ICpackage, the method comprising: placing a frame in a mold wherein theframe has a top surface, a bottom surface, and a top-to-bottom openingtherein; substantially surrounding the frame with a mold compound;attaching a die to the mold compound including positioning the die sothat its periphery lies over the frame; attaching the die to a leadframe of the IC package; and enclosing the die by attaching a window tothe mold compound above the die.
 10. The method of claim 9, whereinsubstantially surrounding the frame with a mold compound is performed bysubstantially surrounding a ceramic frame with a mold compound.
 11. Themethod of claim 9, wherein substantially surrounding the frame with amold compound is performed by substantially surrounding an alloy framewith a mold compound.
 12. The method of claim 10, wherein substantiallysurrounding the frame with a mold compound is performed by substantiallysurrounding an alloy-42 frame with a plastic mold compound.
 13. A methodof making an IC package, the method comprising: attaching a die to amold compound having a frame embedded within it including positioningthe die so that its periphery lies over the frame wherein the frame hasa top surface, a bottom surface, and a top-to-bottom opening therein;attaching the die to a lead frame of the IC package; and attaching awindow to the mold compound above the die, to enclose the die.
 14. Themethod of claim 13, wherein attaching the window to the mold compoundfurther comprises: attaching the window to an alloy window frame; andattaching the alloy window frame to the mold compound.
 15. The method ofclaim 13, wherein attaching the window to the mold compound furthercomprises: attaching the window to a ceramic window frame; and attachingthe ceramic window frame to the mold compound.
 16. A method of attachingan IC package to a circuit board, the IC package comprising a moldcompound having an embedded frame wherein the frame has a top surface, abottom surface, and a top-to-bottom opening therein, a lead frame, a dieattached to the lead frame, a periphery of the die being located abovethe embedded frame, and a window located above the die to allow light toreach the die, the method comprising: placing the IC package inproximity to the circuit board; and attaching the IC package to thecircuit board via a mass reflow process.
 17. The method of claim 16,wherein attaching the IC package to the circuit board comprises heatingthe IC package to above 215° C.
 18. An IC package substantially asherein described with reference to and as shown in FIGS. 5-9B of theaccompanying drawings.
 19. A method as claimed in claims 9, 13, or 16substantially as herein described with reference to and as shown inFIGS. 5-9B of the accompanying drawings.